Precisely following an energy trail isn’t simple. We can broadly speak of energy from the Sun that’s deposited into plants. People can eat plants such as corn to power their bodies or burn plants such as trees to power their technology. These two paths take the plant’s stored energy, use some and release remnants as heat and unspent energy. Yet how do we assess the utility of either of these paths?
Historically, and even today, people use manure as a fuel source in addition to wood. This seems like a good idea. According to the UN’s FAO, people keep almost 1.6B head of cattle and buffalo. A typical cow excretes about 30kg of manure every day. Assume manure has an energy content of 15MJ/kg. Therefore, an estimate of the amount of manure energy produced each year is 2.6e20J. Perhaps surprising, this is almost the same as the total annual human energy consumption from primary fuels.
Perhaps you are thinking that we should all switch to manure as our energy source. We would have lots of food to eat (eg beef steaks) and we would have excess energy to use. Yet, the consumption of manure as a biofuel doesn’t even register. Can you determine why this energy pathway is so neglected?

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